G. Mollenstedt and F. Lenz: "Electron Emission Microscopy" published in Advances in Electronics and Electron Physics, No. 18 (1963), pp 251-329;
L. Wegmann: "Auf dem Weg zum Metall-Electronenmikroskop", published in Prakt, Metallographie, No. 5 (1968), pp 241-263;
G. H. Morrison and G. Slodzian: "Ion Microscopy", published in Anal. Chem., No. 47 (1975), pp. 932A-943A.
Electrically conductive surfaces which emit charged particles can, by means of these particles, be imaged on a magnified scale by the use of a so-called emission lens ("emission microscopy"). The lateral local resolution which is achievable by this method is, in principle, limited by the aperture error of the extraction field in accordance with the relationship: EQU d.sub.el .apprxeq..epsilon./eE.sub.o (1)
wherein:
d.sub.el is the minimum discernible separation between two points on the surface; PA1 .epsilon. is the most probable initial energy of the particles contributing to the formation of the image; PA1 e is the elemental charge; PA1 E.sub.o is the field strength at the surface.
Assuming a given energy distribution of the emitted particles, the resolution can, accordingly, be improved by an energy filtering operation whereby only low energy particles are permitted to take part in the formation of the image. However, this method is accompanied by a loss in intensity of the image. On the other hand, the resolution is improved if the field strength E.sub.o is made greater. In this respect the upper limit is defined by the breakdown strength in vacuum. This amounts to about 10 kV/mm.
The "classical" emission lens, also referred to in the literature as an "emission objective" or "cathode lens", is that of the Johannson type comprising a sample surface, a Wehnelt electrode and an anode. In that arrangement, in order to produce a real image of the sample surface, the field strength at that surface must be substantially smaller than the field strength between the Wehnelt electrode and the anode. The latter is, however, limited by the breakdown strength, so that in fact the field strength at the surface lies substantially below the value, which, according to Equation (1), results in poor local resolution.
In the art of image formation by means of emitting electrodes, this difficulty is overcome in the design of more modern apparatus by the use of an extraction path of maximum permissible field strength in combination with a following magnetic lens or, alternatively, an individual electrostatic lens. Because, in the design of electron emission microscopes, it is possible to use beam energies up to 50 keV, there can still be a working distance of several millimeters between the sample surface and the acceleration electrode even when there is the maximum permissible field intensity at the sample surface.
In image formation by means of emitted ions, which are then mass analysed by a magnetic sector field ("ion microanalysis"), it is possible to use beam energies of up to only about 10 keV if the size of the analyzing magnet is to be kept within reasonable dimensions for handling. Furthermore a working distance of several millimeters is necessary in order to provide the possibility of exciting the sample surface to emit secondary ions by bombardment with a laterally projected primary ion beam. Consequently, when it is desired to use as the emitting optical system, a combination of extraction path and an individual lens, the attainable field intensity at the test surface will then be substantially less than the above mentioned limit of 10 kV/mm.